Diamond is a difficult material to form into complex structures because of its hardness and chemical inertness. Traditionally diamond has been shaped by lapidary techniques such as lapping and polishing using fine diamond grits and powders as the abrasive medium. One typical product of such processes is faceted natural diamonds used as jewellery. Examples of industrial products formed by lapidary processes are polycrystalline diamond plates used as exit windows for high power CO2 lasers. In all these cases, the finished diamond product has large flat surfaces (i.e. extending laterally greater than a few 10 s of micrometers), ostensibly free from surface features which affect the performance.
The use of lapidary processes on diamond has the disadvantage that they result in the surface being processed having a ‘damaged zone’ that extends to a depth roughly equal to the diamond particle size used for processing.
For applications that require fine three-dimensional surface structural features, lapidary methods are not suitable. Liquid chemical etching processes, such as are used extensively in the microelectronics industry are not applicable to diamond because diamond is resistant to almost all chemicals.
High temperature gas phase etching processes have been used to etch diamond. The use of hydrogen-argon-oxygen and hydrogen-argon plasmas at high temperature (>700° C.) to etch substrates prior chemical vapour deposition (CVD) diamond synthesis is disclosed in WO 01/96633. Such an etch preferentially etches damage features associated with the sub-surface damage layer and therefore the roughness Rq is generally significantly increased as a result of the etch process.
In typical reactive ion etching (RIE) processes, large numbers of ions are produced that are accelerated towards the target and physically remove material by sputtering and related processes. The process can have low selectivity between materials and therefore is not always ideal for patterning surfaces. EP1555337 discloses the use of a reactive ion etch (RIE) using an oxygen-carbon fluoride (O2—CF4) gas mixture at a pressure of 1.33-13.3 Pa (about 10-100 mTorr) to prepare mechanically processed single crystal diamond surfaces for CVD growth.
In contrast to RIE, inductively-coupled plasma (ICP) etching is a largely chemical process in which a plasma is used to breakdown the etching gases into a mixture of free radicals (i.e. neutral species) and ions (i.e. charged species). The plasma is remote from the substrate being etched. Between the plasma and the diamond being etched, the vast majority of the ions generated in the plasma are removed. Thus the majority of species that reach the diamond are neutral. The resulting etching is therefore largely chemical (e.g. surface reactions leading to volatile products), rather than physical (e.g. sputtering from the surface by ions from the plasma). Since atoms in a higher energy state in the substrate, such as those in a region with extended lattice imperfections (e.g. a damaged region), are easier to etch, then this type of etch generally preferentially etches the regions of extended lattice imperfections, roughening the surface.
The use of inductively-coupled plasma (ICP) etching with an argon-oxygen gas mixture to pattern natural single crystal diamond is reported in H. W. Choi et al, ‘Properties of natural diamond microlenses fabricated by plasma etching’, Industrial Diamond Review, Issue 2, 2005, page 29, and to pattern polycrystalline diamond made by a chemical vapour deposition process in M. Karlsson et al, ‘Transfer of continuous-relief diffractive structure into diamond by use of inductively coupled plasma dry etching’, Optics Letters, 26 (2001), 1752-1754. UK Patent Application GB 2 281 254 discloses the use of a mixture of oxygen and fluorine containing gases in a plasma etching method for diamond. No disclosure is made about the roughness of the etched surfaces.
ICP etching of gallium nitride (GaN) using an Ar/Cl2 gas mixture is reported in H. W. Choi et al, Journal of Applied Physics, 97 (2005), 063101.